As a silicon-based CMOS device in dimension shrinks to a nanometer scale, the conventional method that improves performance of the device by reducing the dimension is facing the dual challenges of physical and technical limitations. To further improve an operating speed of the device, it is required to use high-mobility channel materials. Under low electrical field, germanium material has a hole mobility four times as great as that of silicon material and an electron mobility two times as great as that of silicon material. Thus, germanium material as a new channel material has become one of the promising developing targets for high performance MOSFET devices by virtue of the higher and more symmetric carrier mobility. However, now the preparation technology of germanium-based MOS device is not mature, and NMOSFET device performance is not desirable. High source-drain series resistance is one of the key factors affecting improvement in germanium-based NMOSFET performance.
As compared with silicon material, in germanium material the N-type impurities (such as phosphorus) have lower activation ratio and diffuse more rapidly, it is adverse to the preparation of shallow junctions. Schottky junctions can effectively overcome the above problems, and hence has become a very promising structure. The Schottky junction are different from the conventional PN junction by adoption of a metal or metal germanide instead of a conventional highly-doped region, and this structure not only avoids the problems of low solid solubility and rapid diffusion of impurities, but also achieves an abrupt junction and a low resistivity. For the Schottky junction, the key factor affecting performance is the carrier barrier height between the substrate and the metal. Furthermore, when the metal is in contact with the germanium substrate, the Fermi level is pinned in the vicinity of the top of the valence band, thus the electron barrier height is great, and it is adverse to improve the Schottky junction performance. The Fermi level pinning effect on germanium surface is caused by the following two factors: one is an interface state formed due to the factors such as dangling bonds and defects on the surface of the Germanium material; and the other is a metal-induced-gap-state (MIGS) generated in the forbidden band of the Germanium semiconductor due to incompletely attenuation of electron wave function of metal in Germanium, according to the Heine theory. To eliminate Fermi level pinning between the germanium substrate and the metal, a dielectric layer is interposed between them, and then on the one hand, the dangling bond on the germanium can be passivated to improve the quality of the interface between the germanium substrate and the metal; on the other hand, the interposed dielectric layer may block the electron wave entering the germanium substrate, thereby reducing the MIGS interface state. At present, the materials used as the dielectric layer includes Si3N4, Al2O3, Ge3N4 etc., but the conduction band offsets of these materials relative to germanium are great, so a larger tunneling resistance may be introduced between the germanium substrate and the metal, and this is adverse to increasing the on-state current of the Schottky junction.
As to the passivation of the germanium substrate surface, the rare earth oxides, such as Y2O3, La2O3, CeO2, etc., are considered to be excellent passivation material for the germanium substrate surface. This is because the stable X—O—Ge bond (X refers to a rare earth metal element such as Y, La, Ce or the like) may be generated at the interface between the germanium substrate and the metal, when the rare earth oxides are in contact with the germanium substrate, the dangling bonds at the germanium surface is effectively passivated, and the quality of the interface is improved.